Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
As mentioned, radio frequency (RF) coils are used in an MRI system to transmit RF excitation signals and to receive MR signals emitted by an imaging subject. Various types of RF coils may be utilized in an MRI system such as a whole-body RF coil and RF surface (or local) coils. Typically, the whole-body RF coil is used for transmitting RF excitation signals. One or more (e.g., an array) surface coils can be used as receive coils to detect MRI signals. Surface coils may be placed in close proximity to a region of interest in a subject and typically yield a higher signal-to-noise ratio (SNR) than the whole-body RF coil. During transmission of RF excitation signals, the surface coils can couple to the whole-body RF coil causing distortion of the excitation field (B1) and affecting image quality. Accordingly, the surface coil (or coils) is disabled, or decoupled, during the transmit (Tx) mode (in which the whole-body RF coil transmits the RF excitation signals) to reduce distortion of the excitation (B1) field. The surface coils are then enabled when the MRI system switches to a receive (Rx) mode to receive the induced MRI signals.
Various decoupling schemes and circuits have been developed for surface RF coils to ensure that the surface coils switch between enabled in the receive mode and disabled in the transmit mode. Developments in MR applications, for example, using the transmit body coils B1 field for calibration, have resulted in more stringent requirements for less B1 distortion during the transmit mode to achieve B1 homogeneity. In addition, faster imaging techniques that have been developed create a need for reduced switching times for RF surface coils between enabled in the receive mode and disabled in the transmit mode. One method to reduce the B1 distortion is to include decoupling circuitry at more points in a surface coil. However, conventional decoupling schemes typically have an associated lowering of the quality factor (Q) of the surface coils and an associated delay in switching times as the number of decoupling points in a surface coil increases.
One conventional decoupling scheme switches an inductor across one or more tuning capacitors of a surface coil. Typically, the switching is accomplished with diodes. A resistor can be placed directly across each switching diode to remove unwanted charge. The resistance value of the resistor used can affect both the quality factor (Q) of the surface coil and the switching speed of the decoupling circuit. Currently, the choice of resistor value can have two conflicting impacts on the performance of an RF surface coil. The use of a higher value resistor favors higher Q of the surface coil in receive mode at the cost of having a longer switching time. The use of a lower value resistor favors shorter switching time at the cost of Q in the surface coil in the receive mode.
It would be desirable to provide a decoupling circuit for an RF surface coil that provides faster switching speeds while maintaining a high Q for the surface coil.